Update 42 trapping rain water with O(1) space

Jay-0331 · Jul 11, 2022 · da25527 · da25527
1 parent baf691d
commit da25527
Showing 1 changed file with 27 additions and 0 deletions.
diff --git a/java/42-Trapping-Rain-Water.java b/java/42-Trapping-Rain-Water.java
@@ -20,3 +20,30 @@ public int trap(int[] heights) {
  }
 
 }
+
+//O(1) space
+class Solution {
+ public int trap(int[] heights) {
+
+ if (heights.length == 0) return 0;
+
+ int l = 0, r = heights.length - 1;
+ int leftMax = heights[l], rightMax = heights[r];
+ int res = 0;
+
+ while (l < r) {
+ if (leftMax < rightMax) {
+ l++;
+ leftMax = Math.max(leftMax, heights[l]);
+ res += leftMax - heights[l];
+ } else {
+ r--;
+ rightMax = Math.max(rightMax, heights[r]);
+ res += rightMax - heights[r];
+ }
+ }
+
+ return res;
+
+ }
+}